1. Field of the Invention
The invention relates to optical lithography field, especially for a lithography pupil shaping optical system and method for generating off-axis illumination mode. The invention is a pupil shaping optical system, which can generate single ring illumination mode and double ring illumination mode. The intensity at pupil plane and the inner and outer diameters of the off-axis illumination mode can be adjusted continuously.
2. Background Information
As a kind of key semiconductor processing equipment, the resolution of optical lithography machine is one of the important specifications to evaluate its performance. From the equations of resolution and depth of focus, it is known that shortening the exposure wavelength and increasing the numerical aperture of the projection lens are effective methods to improve the lithography resolution. But the application of these methods to improve lithography resolution will also decrease the depth of focus and reduce the edge steepness of exposure line edge. Recently, researchers have proposed a variety of lithography resolution enhancement technologies which improve the lithography resolution while ensuring the required depth of focus. These enhancement technologies mainly include off-axis illumination, phase shift mask, optical proximity effect correction and advanced lithography process technology. Among them the off-axis illumination is a kind of commonly adopted lithography resolution enhancement technology.
In an optical lithography machine, the objective of pupil shaping is to obtain various off-axis illumination modes. Usually, a specially designed optical element which modulates the intensity or phase distribution of the incident laser beam is adopted to obtain specified intensity distribution at pupil plane. The diffractive optical element is commonly used to achieve pupil shaping. It modulates the amplitude or phase of the beam incident on it. The specific diffraction pattern, such as annular, dipole, quadrupole or etc, is generated in the far field. The inner and outer diameters of the pattern at pupil plane can be changed by adjusting the focal length of the zoom collimating lens group and the distance between the convex and concave of the axicon pairs. The zoom collimating lens group is composed by a set of lenses and their positions could be adjusted. Based on the lithography simulation results, the selection of the pupil distribution in off-axis illumination is mainly depend on the shape, distribution and size of the mask pattern and the numerical aperture of the optical lithography machine. With the increasement of numerical aperture, the decreasement of the feature size of lithographic pattern and the improvement of its distribution density, a variety of new off-axis illumination modes appear, such as double annular, double dipole and double quadrupole.
The Chinese Laid-Open Patent Applications CN101916045A and CN101916046A disclose two freeform lenses which are used to generate double dipole and double annular illumination modes respectively. In their design the principle is that the incident light deflected by the designed transmission surfaces with specific shapes into specified direction to obtain different illumination modes. But this technique has the following disadvantages: The shape of the curved surfaces is complicated to realize; The intensity distributions of the inner and outer rings and their inner and outer diameters are non-adjustable. Therefore, they are non-flexible and not suitable for different photolithographic mask patterns.